In recent years, biological film treatment processes have been more commonly recognized and used for treating sanitary and industrial wastewaters instead of widely adopted conventional activated-sludge processes.
The basic principle of the biological film treatment process consists in that wastewater flows and circulates around a contacting medium whereupon microorganisms accumulate and decompose organic substances included in the wastewater through oxidation, resulting in the removal of waste products.
The most important design subject of the biological film treatment process is a contacting element upon which bacterial solids accumulate. Conventionally, most attempts have aimed at maximizing surface area for bacterial growth. For example, a known rotating biological contactor employs a plastic contacting element of honeycomb structure having a surface of 200 m.sup.2 /m.sup.3.
However, this plastic contacting element has not yet proved to be suitable for bacterial growth and, on the contrary, it hardly allows accumulation of bacteria due to its slippery surface. Furthermore, upon thickening of the biological layer on the plastic element, the bacteria within the layer become anaerobic and simultaneously a large amount of the layer, having a thickness of about 25 mm or more, may fall off from the plastic element. This decreases treatment efficiency. Such plastic contacting elements have the further disadvantage that they may become clogged unless they are spaced by 50 mm or more.
When purifying wastewater using microorganisms, a great problem involves how to treat inactive bacteria, that is, dead bacterial bodies. In particular, the pores of a biological film provided for attachment of bacteria may become clogged with dead bacteria, whereby it becomes necessary to replace the film with a new one. This requires maintenance and expense. For this reason, equipment for treating wastewater with biological films has only been adopted in limited circumstances by countries, local public organizations and big companies and are essentially unusable by individual people. However, since pollution of rivers and lakes is limited by sanitary waste discharges, it would be a significant achievement if every family, individual and/or medium- and small-sized enterprise could treat their liquid wastes using biological films.